We have found that nerve growth factor (NGF) causes rapidly onsetting changes in the morphology, motility and rate of extension of growth cones. We propose to study the mechanism of this phenomenon with cultured sympathetic-neuron-like pheochromocytoma cells (PC12) and sympathetic neurons. The activity and shape of the growth cone and dependence on the presence of NGF will be characterized qualitatively from time-lapse video observations made with the phase contrast microscope. Features will be quantified by scoring cells in fixed cultures. Dose-response and specificity of NGF effects will be determined. Requirements for cyclic nucleotides, calcium ion and internalization of NGF in the mediation of NGF-induced events at the growth cone will be examined by administration of effector and antagonist molecules to the cells. It will be possible to interfere specifically with the processes being studied by microinjection of materials into the cytoplasm. Microinjection will also be used for introduction of functional fluorescent derivatives of cytoskeletal proteins into cells. The effect of NGF on these fluorescent proteins at growth cones will be observed using time-lapse methods. Studies with the transmission electron microscope will be used to link high resolution views of the cytoskeleton with the motile forms of the growth cone seen in culture and will form a bridge to future work on extension of the neuritre tip induced by NGF. Since, in an outgrowing axon, the growth cone would be the site of reception of trophic signals from peripheral or target tissues, these studies are directly concerned with the mechanisms of proces outgrowth and thus with possible therapeutic interventions in nerve regeneration.